1. Field of the Invention
The present invention relates to a battery pack for a computer system including static memory to retain battery operating parameters and charge information and a real time clock for measuring periods of non-use.
2. Description of Related Art
Computers are often needed in locations where AC power is not available. Rechargeable batteries are typically used as an alternative source of power, such as nickel-based batteries including nickel cadmium (NiCad) and nickel metal hydride (NiMH) batteries, which are capable of providing power to a portable or pen-based computer for several hours. Since rechargeable batteries have a limited cycle life, it is desirable to maximize the life of and to obtain the maximum power from each battery during every discharge cycle. To achieve these goals, it is necessary to fully and efficiently charge the battery without causing temperature abuse as a result of overcharging.
Many battery chargers presently in use do not efficiently charge a rechargeable battery, thereby limiting the useful life of the battery. The total charge capacity of a battery, typically referred to as the amp-hour (Ah) rating or otherwise called the milliamp-hour rating (mAh), tends to change over time and usage. For example, the charge of a battery typically increases and then decreases over time and usage until the battery eventually will not hold a charge and must be replaced. In particular, NiCad batteries exhibit a memory effect causing lower capacity after multiple partial charge and discharge cycles have occurred. Some battery chargers monitor the temperature and voltage to avoid fast charging when the battery voltage is too low or when the battery temperature is not within an acceptable fast charging temperature range. Examples are disclosed in U.S. Pat. Nos. 5,136,231 and 5,382,893. These and most other known battery chargers do not have the capability to accurately determine the charge level and the total charge capacity and thus may still overcharge the battery, thereby further reducing its useful life.
The primary reason for temperature abuse and overcharging of nickel-based batteries was the difficulty of determining the charge level since the voltage is substantially the same regardless of the charge level. Furthermore, if the battery was removed and then later re-installed, the system had no way of determining the charge history and the remaining charge level of the battery. Some methods estimated the charge level by monitoring the charge and discharge current through the battery over a period of time, but these methods still assumed an initial level of charge. These methods proved inaccurate when a partially discharged battery was used. Although the level of discharge may be measured for a single dedicated battery in a particular system, the battery is typically interchangeable with another in many systems, so that the system has no way of determining the charge level of a given battery. Also, rechargeable batteries tend to discharge during periods of non-use, referred to as self-discharge, so that a system which was shut off for a period of time and then powered up could not determine the amount of self-discharge that had occurred.
One particular technique used in personal computers, known as "Battery Watch", estimates the amount of charge remaining based on the lapsed time of charge and discharge, and converts this to the useful time remaining. This technique has proven unreliable since it assumes that the total charge capacity of a battery does not change over time. Further, this technique fails to keep track of the charge-in and charge-out and does not account for the self-discharge.
One approach to solve most of these problems is disclosed in U.S. Pat. No. 5,315,228. A battery pack was disclosed which included a nickel-based battery, a microcontroller circuit and memory. Since the microcontroller circuitry was contained within the battery pack, it was capable of monitoring the battery charge status at all times by continually sampling the charge and discharge current through the battery as well as the battery voltage and temperature while charging. The microcontroller used the battery charge status information to accurately calculate the total charge capacity and the remaining charge level in the battery. The memory stored the charge information and a communication means was provided to transfer the stored information to a computer system when the battery pack was installed in the computer.
The advantages of this system include the ability to recalculate the total capacity of the battery over time, to provide a fuel gauge to continually measure the remaining charge level of the battery at any given time and the ability to measure the battery self-discharge during periods of non-use. The local battery circuitry also controlled battery charging and used the above calculations to determine the optimal point of fast charge termination for the battery. Essentially, all charging functions and control were provided in the battery pack, alleviating the need for the computer system to determine when a new battery was placed into the system and how to charge the battery.
It has been discovered that the use of microcontroller circuitry in the battery pack is relatively expensive and also reduces battery shelf-life since the microcontroller circuit continuously draws a significant amount of current from the battery during periods of non-use. The power consumption is particularly relevant when the size and charge capacity of the nickel-based battery itself is reduced for purposes of convenience and cost. The microcontroller circuitry consumes valuable space and adds cost since it must be purchased with each battery pack. Computer designers must seek new ways to maximize power consumption efficiency and valuable space to meet the needs of the computer market. This is particularly pertinent with the advent of pen-based computer systems where size, cost and power usage are critical.
It is desirable, therefore, to keep track of the particular battery statistics and charge status, including total capacity over time as well as the instantaneous charge level at any given time, taking into account periods of self-discharge and reduced capacity. It is further desirable maximize the useful power of the battery while keeping the size of the battery as small as possible.